The terms oxymethylene polymer and polyoxymethylene as used interchangeably herein are meant to include oxymethylene homopolymers and diethers and diesters as well as oxymethylene copolymers and terpolymers, which are oxymethylene polymers having at least 60 percent recurring oxymethylene units and at least one other unit derived from a monomer copolymerizable with the source of the oxymethylene units.
Oxymethylene polymers having recurring --CH.sub.2 O-- units have been known for many years. They may be prepared for example, by the polymerization of anhydrous formaldehyde or by the polymerization of trioxane, which is a cyclic trimer of formaldehyde, and will vary in physical properties such as thermal stability, molecular weight, molding characteristics, color and the like depending, in part, upon their method of preparation, on the catalytic polymerization technique employed and upon the various types of comonomers which may be incorporated into the polymer.
Polyoxymethylene molding compositions have been in commercial use for many years. They have application in a wide variety of end uses, e.g., automobile applications such as bumper extensions and instrument panels, plumbing supplies such as valves, shower assemblies, flush tank components, and pipe fittings, tool components such as screw driver adaptors, and household and personal products, such as quick boiling electric water kettles, clothes hangers and combs.
Polyoxymethylenes have a highly favorable spectrum of physical properties which are responsible for their wide commercial acceptance. However for certain molding applications, e.g., exterior automobile body parts, molding compositions that impart higher than usual impact strength to articles molded from them are highly desirable.
It is known in the art to add a multi-phase composite interpolymer to a thermoplastic polymer such as a polypropylene terephthalate, a polybutylene terephthalate, a polyethylene terephthalate, a polycarbonate, a polyvinyl halide, an acrylic polymer or an oxymethylene polymer to increase the impact properties of the thermoplastic polymer. Examples of such compositions may be found in U.S. Pat. Nos. 3,843,753, 4,086,296, 4,096,202, 4,180,494 and 4,200,567, and in commonly assigned application Ser. No. 765,014, filed Aug. 9, 1985, now U.S. Pat. No. 4,665,126.
Also known in the art are polymer blends of 40 to 99% of a thermoplastic polyurethane, 1 to 60% of a thermoplastic polymer which can be a polyoxymethylene, and 0.5 to 10% of an acrylic polymer as a processing aid. The blend is stated to provide a more homogeneous melt and enhanced hot metal release and lubricity.
While these prior art compositions are useful, a polyoxymethylene molding composition exhibiting toughness would be highly desirable. The present invention provides such a composition.